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[♩INTRO]
We often think of the ocean as, like, a very serene, lovely place to relax.
But that’s only because we’re oblivious to the racket that is going on underwater.
From sea urchins rasping on rocks to whales bellowing,
there’s a cacophony of sound in the ocean — and for good reason.
Water’s densely-packed molecules make it a great sound conductor.
In fact, sounds travel more than four times faster and farther in water than in air.
But, don’t expect to hear that from the beach.
When sound waves from underwater hit air at the surface,
they reflect back into the depths.
And even when we’re swimming, our ears aren’t adapted for underwater hearing.
That said, the incredible sound transmission underwater makes the ocean
a great place for marine animals to communicate,
or even use sound to defend themselves.
And while most people are familiar with whale songs and dolphin chatter,
there are some unlikely culprits, including these six.
First up are, maybe unexpectedly, seahorses.
You might not think of them as loud,
but they make two kinds of sound that are hard to ignore.
The first is a growl.
It’s only been recorded in captive seahorses,
but researchers think this warning sound is likely used in the wild as well.
It comes from the animals using muscle contractions to vibrate their bodies
particularly, their cheeks.
And at 115 decibels, the growl is as loud as an ambulance siren.
Researchers think the point of the sound might be to scare away danger
and give the seahorses a chance of escape.
I would be afraid of a 115 decibel growl,
even if the thing that made it was like this big.
Specifically, if a predator has captured a seahorse, these low, long pulses,
along with the body quivers, might make them let go.
That’s only half the story with these animals, though
because when seahorses are courting,
they also make another distinctive noise: loud clicking sounds.
They’re not as loud as their growling, but at 105 decibels,
the clicking still rivals a car stereo at max volume.
The seahorses make the sounds with their skulls.
As an amorous seahorse rubs its own skull bones together,
it generates the clicks
each one a low-frequency sliding sound followed by a higher-frequency knocking.
Males and females even make duets of headbone clicks
during their complex array of courtship behaviors.
And while it’s not clear how this behavior evolved,
a study found that female seahorses click more often than males.
So it may have something to do with sexual selection,
where males choose a female based on how “clicky” she is.
Next, many ocean fishes can make some impressive sounds of their own,
using their swim bladders: gas-filled organs that help a fish maintain buoyancy.
A swim bladder is like a personal balloon that expands
and contracts as it lets air in and out.
But some fish also use these big balloons for their acoustic properties
like a group called toadfishes.
The male toadfish guard their nests and, during the night,
they use muscles around their swim bladders to vibrate them,
creating a call to lure in females.
[toadfish grunts]
This call involves a grunt like a foghorn followed by higher frequency boops.
And it can reach 138 decibels, which is louder than a jet taking off.
Each male toadfish has a signature style and voice,
which is a key part of using sound for communication.
Like, their grunts vary based on the number of boops, length of boops,
and pauses between them.
And these signals might convey information that helps females decide
whether he’s worthy of her attention.
Researchers hypothesize that the males might be signaling things
like their age, health condition, or level of motivation to mate.
Or maybe even their size!
For instance, a 2010 study on Lusitanian toadfish found that larger males
vibrated their swim bladders faster, thanks to bigger and healthier muscles.
Now, one toadfish is loud enough on its own.
But when fishes with sonic swim bladders group up,
the volume goes through the roof.
Just ask the Gulf corvina fish.
Every spring, all the adults congregate in one spot:
the Colorado River Delta in the Gulf of California.
Unlike the toadfish, who spread out into different burrows,
the corvinas cluster into a huge swimming, spawning group.
Spawning aggregations like this are thought to increase breeding success
by concentrating males and females in a known place at a known time,
based on natural cues.
Like, in this case, Gulf corvina might be taking advantage of the extreme tides
in the area at certain parts of the lunar cycle,
which could wash their eggs out to sea.
Guided by the tides and moon cycles, adults arrive
within a few days of each other.
And that’s when the shouting starts.
[gulf corvina grunts]
Biologists compare the gathering to a cocktail conversation,
with males trying to be heard by females over the din.
In a school of thousands or millions of fish, the male corvinas generate
177 decibels of sound — something louder than a shotgun.
In fact, these sounds are so loud that they lead to overharvesting.
They actually make fishing boat hulls buzz,
sometimes like a massive swarm of bees.
Which while it might be super startling at first, does eventually provide
a really great way for commercial fishers to hone in on their targets.
Now, communication is a huge part of underwater sounds.
But lots of ocean animals also use sound for defense — like spiny lobsters.
Lobsters are arthropods, in the same large group as insects.
And many arthropods make sounds by rubbing
parts of their hard exoskeletons together, including grasshoppers and locusts.
But, spiny lobsters have taken a novel approach.
They rub a piece of soft tissue called the plectrum
at the base of their antennas against an area by their eyes.
As the plectrum sticks and then slips on this area,
it works kind of like a sticky violin bow and makes a rasping sound.
The ability to use soft tissues might be especially helpful
to a recently-molted lobster that’s just outgrown and shed its old exoskeleton.
During this time, the animal is especially vulnerable
since its new body covering hasn’t hardened yet.
But even though it’s lacking that defense,
it can still make the same warning sound as everyone else.
Also, if you’re thinking that soft tissue can’t generate that much noise,
you’d be surprised.
At 150 decibels, this lobster’s warning is as loud as a fighter jet.
And like many animal warning sounds,
it’s abrasive enough to help drive off predators that get too close,
along with other defense behaviors like lunging or flipping their tails.
Also, as a nice bonus,
this sound fades into the loud background noise pretty easily, too
so it can deter predators, but also doesn’t draw much attention to the lobster.
Next, even an ocean worm can make itself heard.
Snapping worms live inside sponges more than 100 meters underwater,
off the coast of Japan.
They have see-through bodies, with tentacles surrounding a circular mouth.
And when it comes time for the worms to defend
the narrow sponge chambers they call home, their mouths come in handy.
The worms will run into each other, mouth-first,
and then will burst away with a startling popping sound.
It’s hard to figure out exactly why the sound evolved.
Like, it might have been to startle the competitor,
or could just be a side effect of the attack.
But either way, it’s definitely noticeable:
At 157 decibels, the sound is louder than the fans
at the loudest sports stadiums in the world.
Scientists recording these interactions are also still figuring out how the worms
make such a loud sound, but one idea is that it has to do with part of their
digestive tract, and a process called cavitation.
Cavitation happens when part of a liquid experiences a big temperature
or velocity change, which causes a dramatic change in pressure.
It’s so dramatic that the liquid can temporarily go from a liquid to a gas,
forming a bubble.
Then, when that bubble encounters the rest of the liquid,
the gas turns back into a liquid with an intense pop.
In the case of the worms, scientists think they use their muscles
to block off a piece of their digestive tract — one that’s full of water.
And then, they use other muscles to expand that little chamber,
changing the pressure in there so fast that the water becomes a gas.
And finally, they open up that part of their digestive tract
and let the chamber collapse.
When they do, that gas converts back into a liquid,
and the water shoots out of their mouths at the other worm
creating the pop, and also sending them flying backward.
Now, cavitation isn’t unique to these worms.
If you’ve ever been around a loud boat propeller, you’ve probably experienced it.
But the fact that these little worms make so much sound,
and probably with their little internal water guns, is pretty special.
Still, when it comes to cavitation,
they can’t beat the pinky-sized snapping shrimps.
These animals make repetitive cracking sounds
that punctuate the tranquility of coral reefs.
In fact, the cracking is so constant that in World War II,
submarines took advantage of it to hide from sonar
a technique that uses sound waves to identify objects.
[shrimp crackling]
The shrimp make this racket by snapping their claws shut,
forcing out a jet of water so fast, it forms a low-pressure zone behind it.
The water in that zone changes state into a gas,
and then it’s basic cavitation all over again.
When that gas runs into the water around it,
it converts into a liquid again, and you get a pop.
Except this time, it releases so much energy that it creates a flash of light,
along with a shockwave that kills nearby fish.
And at 220 decibels, the collapsing cavitation bubbles are even louder
than the takeoff of the Saturn V rocket, which carried astronauts to the Moon.
Researchers think this mechanism may have evolved for feeding,
but today, it’s also used to spar with rivals.
Because if evolution gives you a weapon like that, you might as well use it.
The diversity of underwater sound is incredible, and speaks to how much animals
have evolved to take the water they live in and use it to their advantage.
But also, be thankful human ears don’t work so well down there!
These sounds are so loud, that if our ears were evolved to actually hear them,
they could easily damage nerves or even bust eardrums.
So if you can, it’s best to put on a snorkeling mask and just enjoy the view
or the feel of the water without eavesdropping on anyone’s conversations.
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[♩OUTRO]